When a cursor on a display is moved by an input device, such as a keyboard or a mouse, the cursor moves along pixels on a display according to a mapping from the input device to the coordinate system of the display. For example, when a mouse is moved on a tablet from one position to another, a relative position in terms of a coordinate system of the tablet is determined by movement of a roller under the mouse relative to the tablet. This movement is mapped directly to pixel coordinates on the display. When the cursor is moved with keys on a keyboard or keypad, an arrow button directs the cursor to move by some number of pixels, typically along a row or column on the display.
In computer aided design (CAD) systems, a user builds models which are displayed on a video display terminal. Such models can include two dimensional representations of three dimensional surfaces. When a CAD user moves a mouse, the cursor moves along pixels of the display without regard to the model that is displayed. Consequently, the positioning and movement of the cursor does not provide the user with any substantive information about the model.